A long-standing problem that electrochromic and optoelectronic devices share with other thin film device structures is that of environmental degradation. Attempts to eliminate this problem by encapsulating or coating such devices have generally proven unsuccessful.
One existing process for creating a barrier coating involves using an energetic sputtering processing to create a graded barrier film. In particular, the sputtering process is based on the use of oxygen and nitrogen reactive ions in an alternating sequence to create a graded barrier file including metal oxide layers and a metal nitride layer. For example, a metal target is bombarded by energetic argon and reactive gas plasma in a vacuum environment and sputtered material is deposited on a substrate. The coating deposited on the substrate is no longer metal but a metal oxide or metal nitride. Metal oxides are preferred to metals for use with some thin film devices because metal oxides like AL2O3 or AlN are transparent while metals are too opaque for light transmittance as required by some thin film devices.
However, the sputtering process creates films that have internal stress. The thicker the film, the higher the stress. Reactive sputtering from an Al target results in dense Al2O3 layers that act as the barrier functioning layers and an AIN layer that acts as the stress release layer. In other words, this existing process relies on a sputtering process to create a thicker metal nitride layer to support metal oxide layers, thereby reducing inherent cracks in the metal oxide layers.
However sputtering is an energetic deposition process that generates heat on the substrate surface such that it is not appropriate to deposit a film on plastic substrates. The high energetic nature of the sputtering forms high packing density rigid films that easily crack on flexible substrates. In this process, solid target material is bombarded with high energetic gas plasma to create sputtered particles from the target. These particles react with reactive gas and the metal particles oxidize to form a solid oxide film on the substrate. The higher the bombardment of ion energy, the higher the density of oxide film, which increases barrier performance; however, even the dense oxide film has contains numerous voids that still allow for oxygen and humidity penetration.
Thus, depositing a metal oxide, like large marbles in a jar which could freeze upon touching, there becomes a high amount of void space for ingress of foreign gases to move through. Therefore, the vacuum deposited metal oxide, itself, is a poor environmental barrier layer, thereby requiring other layers and increased thickness to make up for its porosity. This is this why some existing processes for creating a barrier film rely on the combination of several vacuum deposited metal oxide layers and a nitride layer to provide a barrier protection, i.e., overlapping multiple layers metal oxide/nitride layers having respective porosity, bring down the over porosity of the overall layers.
Further, sputtering is an energetic deposition process that generates heat on the substrate surface, which is not compatible with depositing a film on plastic substrates as plastics require a low temperature process. Further, the high energetic nature of the sputtering forms high packing density rigid films that easily cracks on flexible substrates.
These existing process relies on a growing layers of the graded barrier film using oxygen plasma and in a controlled environment, i.e., in a reaction chamber of the sputtering device, which is a time consuming process and adds cost to the process as the environment for growing the metal oxide and metal nitride layers needs to be carefully controlled.
Further, the sealing problem is compounded for device configurations that not only require protection but also call for a transparent top layer or layers—e.g., a transmissive mode electrochromic device (ECD) formed on a transparent or opaque substrate. Also, applications of ECD require flexible plastic substrates, which in turn require a flexible barrier coating. Further, plastics are also permeable for oxygen and humidity. Therefore, when a plastic substrate is used for ECD, barrier coatings must be used on both sides of the ECD or other electronic devices.
Another common thin film deposition process is electron beam, commonly called e-beam deposition. In this technique an electron-beam is focused on a deposition material that thermally evaporates it, and vapor phase condenses on the substrate to form a solid film. The density of the film can be control by process conditions. In this technique reactive deposition is also possible by using reactive gas during process. Ion assisted e-beam deposition provides denser films. Depending on the pore size, porous films are still permeable for oxygen and humidity.
Solutions to the sealing problem is also harder to find if the device to be protected cannot withstand high processing temperatures required by some coating processes, described above. An example of this is a transmissive mode ECD formed on a polymeric substrate or supporting member.
Another existing solution relates to polymeric overcoats, such as various poly-para-xylylene coatings sold under the trade name of parylene that have been widely used for environmental protection. Although these are deposited at relatively low temperatures and provide smooth transparent coatings, they are not adequately effective against preventing long term ECD degradation both because the parylene films are somewhat permeable to water and oxygen and because they degrade when exposed to ultraviolet radiation.
Thus, the existing processes fail to provide an adequate protective coating that can be deposited on an ECD, optoelectronic, or other thin film device, at relatively low process temperatures and stress free flexible coatings.